The present invention relates to an X-ray apparatus with a stabilized tube current.
The typical picture data formed by the X-ray apparatus are X-ray photograph by direct photographing, X-ray photograph and X-ray cineradiograph by the indirect photographing using an image intensifier, tomograms by the computed tomography scanner, and the like. To improve a quality of the data, it is very important to stabilize the output of an X-ray source as a data source. Usually, an X-ray tube is used for the X-ray source. Conditions required for obtaining a stability of the output of the X-ray tube are that a tube voltage applied between the anode and the cathode of the X-ray tube is stable, and that the heating of the filament of the X-ray tube is stable.
In the prior measure taken for stabilizing the tube voltage, an AC power from a three-phase of power source is boosted to a higher voltage power by a transformer, the boosted power is full-wave rectified, and the pulsate component contained in the rectified power is absorbed by an electron tube inserted in series in the high voltage section, thereby to form a substantial DC power voltage. The DC voltage is then applied between the anode and the cathode of the X-ray tube.
For stabilizing the heating of the filament of the X-ray tube, a power for heating the filament must be kept constant. One of the methods to realize this is that a rectangular wave AC power is full-wave rectified into a DC power and the DC power is applied to the filament of the X-ray tube. According to this heating method, the supplied power little varies with time, realizing the stabilization of the filament heating.
The X-ray tube is provided with a focusing cap around the filament for focusing electron beams at a point. The filament is electrically connected at one end to the focusing cap, keeping potential of the filament equal to that of the focusing cap.
In the filament heating stabilizing method, a potential difference resides between the other other end of the filament and the focusing cap. But, the potential difference has no variation with time, so that no heating variation arises from the potential variation. Although this method is free from such potential variation problem, it has another problem that the tube current has an influence on the heating of the filament. This will be described below.
One end of the filament is connected at the cathode bus of a high voltage generating circuit. When the filament is heated with DC current as in the above case, a direction of the filament heating current is unidirectional. When the cathode bus is connected to the high potential side of the filament, the filament heating current and the tube current are opposite in the direction of their flows. Accordingly, the filament heating current is cancelled by the tube current, so that the heating effect of the filament is reduced by the amount of the filament current. As a result, the tube current decreases with time. On the other hand, when the cathode bus is connected to the low potential side of the filament, the filament heating current and the tube current flow in the same direction. Accordingly, the tube current is additively superposed on the filament heating current, so that the heating effect of the filament is increased by the amount of the superposed current. Accordingly, the tube current increases with time. As described above, the tube current changes according to the polarity of the voltage at the cathode of the X-ray tube. Consequently, an amount of X-ray radiated from the X-ray tube changes with the change of the tube current, decreasing the stability of the resultant X-ray data.